The balanced chemical equation for the reaction between pcl5 and water is given below. if 3.45 moles of hcl are produced, how many moles of water reacted?
pcl5 + 4h2o -> h3po4 + 5hcl

The coefficients of the substances give you the ratio of the number of moles.
The 4 before the H2O and the 5 before the HCl tell you that for every 5 moles of HCl created, 4 moles of H2O had to react.

Therefore:

can be used to find the moles of H2O that react.